In−situ observation for RH−dependent mixing states of submicron particles containing organic surfactants and inorganic salts
Abstract. Aerosol mixing state plays an important role in heterogeneous reactions and CCN activity. Organic surfactants could affect aerosol mixing state through bulk−surface partitioning. However, the mixing state of surfactant containing particles remains unclear due to the lack of direct measurements. Here, in−situ characterizations of mixing state for 20 kinds of submicron particles containing inorganic salts (NaCl and (NH4)2SO4) and atmospheric organic surfactants (organosulfates, organosulfonates, and dicarboxylic acids) were conducted upon relative humidity (RH) cycling by Environmental Scanning Electron Microscopy (ESEM). As RH increased, surfactant shells inhibited water diffusion exposing to inorganic core, leading to notably increased inorganic deliquescence RH (88.3−99.5 %) compared with pure inorganic aerosol. Meanwhile, we directly observed obvious Ostwald ripening process, that is, the growth of larger crystals at the expense of smaller ones, in 6 among 10 NaCl−surfactants systems. As a result of water inhibition by surfactant shell, Ostwald ripening in all systems occurred at RH above 90 %, which were higher than reported RH range for pure NaCl measured at 27 ℃ (75−77 %). As RH decreased, 8 systems underwent liquid−liquid phase separation (LLPS) before efflorescence, showing a strong dependence on organic molecular oxygen−to−carbon ratio (O : C). Quantitatively, LLPS was always observed when O : C ≤ 0.4 and was never observed when O : C > ~0.57. Separation RH (SRH) of inorganic salt−organic surfactant mixtures generally followed the trend of (NH4)2SO4 < NaCl, which is consistent with their salting out efficiencies reported in previous studies. Phase separations were observed after efflorescence for systems without LLPS. Our results provide a unique insight into the consecutive mixing processes of the inorganic−surfactant particles, which would help improve our fundamental knowledge of model development on radiative effect.
Chun Xiong et al.
Status: open (until 19 Jun 2023)
- RC1: 'Comment on egusphere-2023-849', Anonymous Referee #1, 25 May 2023 reply
Chun Xiong et al.
Chun Xiong et al.
Viewed (geographical distribution)
The research provides direct observation and analysis of the dynamic mixing state and phase transitions of submicron particles containing inorganic and surface-active organic constituents in response to relative humidity (RH) cycling. The research also conducted on the interaction between phase transitions of aerosol particles and related hygroscopicity, and CCN activity which could make a significant contribution to the field. This approach allows for a comprehensive understanding of the dynamic evolution of inorganic-organic particles under fluctuating atmospheric conditions. This study fits within the scope of the journal Atmospheric Chemistry and Physics. However, after carefully examining and revising this article in light of the comments below, it could be considered for publishing in ACP.
- In experimental section, it's important to ensure the ESEM reproducibility. It's unclear from the conclusion if the experiments were repeated and the results were consistent. Further verification in this aspect might be needed. Also, how did you determine the RH step from (a) to (c) of Figure 3? This progress occurred a really narrow range within 0.8% RH. Does it make sense to determine the decimal point of RH range because the authors used 2-3% RH/min condition for RH changes? Furthermore, in Figure 4, if you change the brightness and contrast of the images, do you still obtain same results?
- Please provide the size range of the aerosol particles investigated during the experiments and discuss/compare the size effect with previous study of Freedman et al. in the result section. A figure would be helpful.
- The authors used different temperature range of 0.1 – 25 ◦C in the experiments because temperature has negligible influence on the LLPS of AS-organic and NaCl-organic particles. Is this statement still valid for the submicron particles? Please discuss it.
- The authors showed that LLPS always occurs when the O:C ratio is 0.4 or below, and never when the O:C ratio is higher than ~0.57. This is inconsistent with previous studies (Bertram et al., 2011 ACP; You et al., 2013 ACP; Song et al., 2012 GRL). Thus, the authors should make a careful comparison with the literature and make a clear conclusion. I recommend that all experimental data points including the previous studies should be shown in Figure 5.
- The study also observed that inorganic salt-surfactant systems without LLPS undergo solid phase separation after efflorescence, demonstrating distinct separated phases (pg. 14). This is interesting and pretty new. Thus, I suggest to show the result in the main text and expand the discussion. Figure S2 should be clearer.
- The manuscript has several instances where grammar and language use could be improved for clarity. This includes sentence structure, punctuation, and the use of certain phrases.
- Lines 132 – 133: The sentence needs to be clearer. Please expand the discussion.
- Ensure that the authors used consistent terminology throughout the paper. For example, the terms "inorganic salts-surfactant systems" and "inorganic−surfactant particles" appeared to refer to the same thing, and using them interchangeably may cause confusion.
- Implication: Strengthen the conclusion by making a clear connection between the results and the larger implications for the field of atmospheric chemistry and physics. Providing a more thorough discussion on how your findings could help reduce the uncertainty of model estimation on the global radiative effect could be beneficial.